The present invention generally relates to lasers and more particularly to a laser apparatus including a fiber oscillator.
An ultrafast fiber amplifier is disclosed in U.S. Pat. No. 7,113,327 entitled “High Power Fiber Chirped Pulse Amplification System Utilizing Telecom-Type Components,” which issued to Gu et al. on Sep. 26, 2006, and is incorporated by reference herein. This system follows a traditional approach towards generating high pulse energies from fiber lasers. The guidelines being outlined in the publication of A. Galvanauskas and M. Fermann, “Hybrid Diode-Laser Fiber-Amplifier Source of High-Energy Ultrashort Pulses,” Optics Letters, Vol. 19, No. 14, 1043 (1994), state that high peak intensities inevitably lead to strong nonlinear effects and pulse breakup. The publication further states that the only way to avoid this problem is to maintain sufficiently low peak powers in the amplifier through the use of stretched or chirped pulses. Fiber laser design has not deviated from those guidelines, avoiding nonlinear optical effects and pulse break up through the introduction of pulse stretching optics before power amplification stages. While staying within these guidelines, which require complexity in the form of multiple amplification stages, commercial amplified fiber laser sources now reach tens and even hundreds of micro-Joules per pulse. These sources therefore use multiple stages of amplification to separate the gain into stages to make it more manageable, chirped pulse amplification stretching the pulse by five to six orders of magnitude, and the use of large mode area fiber in order to minimize the peak intensity within the fiber. Minimizing peak intensity is used in order to minimize nonlinear optical processes which have been considered detrimental by causing self-phase modulation, intensity induced dispersion, and pulse break up.
The publication M. Horowitz et al., “Noiselike Pulses with a Broadband Spectrum Generated from an Erbium-Doped Fiber Laser,” Optics Letters 22, 799 (Jun. 1, 1997) teaches away from erbium-doped fiber lasers. The first page of this publication states that “the power of such sources is limited” and “a pulsed erbium-doped fiber laser . . . generates a train of high-intensity, broadband, noiselike pulses.” Generally, optical “noise” is undesirable and to be avoided. The goal of Horowitz is to produce a short coherence length light source and not an efficient laser source. Furthermore, Horowitz comments in the last column: “Our laser cannot support short pulses because of the strong positive dispersion and the significant birefringence, which introduces significant PDD” (polarization-dependent delay).
The first page of the publication B. Ortac et al., “200 nJ Pulse Energy Femtosecond Yb-Doped Dispersion Compensation Free Fiber Oscillator,” Proc. of SPIE, Vol. 6873 (2008) teaches the difficulties with power scaling mode-locked fiber lasers “[m]ainly, due to the tight confinement of the light over considerably long lengths nonlinear effects, mainly Kerr-nonlinearity, avoid self-consistent pulse evolution inside a fiber laser resonator and hinder the pursuit of higher pulse energies from mode-locked fiber lasers. Besides the necessary balance between dispersion and nonlinearity, which can be supported by spectral filtering, the overdriving of the effective saturable absorber can arise as a further energy scaling restriction.” This conventional oscillator uses a 51 cm long large-mode-area fiber with an outer width of 1.4 mm, which is essentially inflexible.
The publication of V. L. Kalashnikov and A. Apolonski, “Chirped-Pulse Oscillators: A Unified Standpoint,” Physical Review A, Vol. 79, 043821 (2009), second column, describes the theory of high-power oscillators and indicates that “energy scaling requires a large negative” net-group-delay-dispersion, “the soliton obtained has a large width, and . . . it is not compressible linearly” because “the peak power P0 has to be kept lower than the threshold value Pth in order to avoid soliton destabilization.” Thus, “one can estimate the maximum attainable energy as E=2PthT,” where T is the soliton width. What Kalashnikov and Apolonski have failed to recognize, is that high intensity pulse trains that are desirable for a number of commercial applications can be obtained from lasers that do not avoid soliton destabilization. In other words, this publication followed the conventional literature in fiber laser design and teaches away from exploring regimes outside single soliton stability. The use of an all-normal-dispersion femtosecond fiber laser design, introduced by A. Chong et al., “All-Normal-Dispersion Femtosecond Fiber Laser,” Optics Express, Vol. 14, No. 21, 10095 (2006), discusses the need to keep intracavity dispersion in the range of 0.04 to 0.10 ps2 in order to obtain femtosecond pulses from a fiber oscillator.
In accordance with the present invention, a laser apparatus includes a fiber oscillator. In another aspect, an Ytterbium (Yb) doped fiber is employed. Another aspect provides an unamplified laser pulse emitted from an Yb fiber oscillator having a repetition rate less than 10 MHz and a pulse energy greater than 100 nJ. In still an additional aspect, the entire laser includes a flexible fiber, with at least one section greater than 10 m, and more preferably greater than 100 m, which is capable of being looped with an outside loop diameter less than 150 mm, and more preferably less than 125 mm. Another aspect provides for a fiber oscillator with passive optical fiber lengths of at least 10 m, and more preferably more than 100 m while having repetition rates less than 10 MHz. A further aspect employs an oscillator design that contains extremely high positive dispersion (greater than 1 ps2), and/or an oscillator that exceeds the threshold of soliton stability by design. Yet a different aspect uses a fiber oscillator to produce discrete femtosecond sub-pulses clustered together in a time period less than 200 fs, without amplification and/or pulse shaping. A method of using an ultrafast laser apparatus is also provided.
The laser apparatus of the present invention is advantageous over traditional devices. For example, greater laser pulse energy can be emitted with lower repetition rates, in compact portable units, and at significantly lower costs than multi-stage amplified systems. The flexible nature of the flexible gain and passive fibers used allows for a very long fiber to be tightly wound, yet providing a high energy pulse output without an additional expensive and heavy amplifier, thereby fitting within the portable unit. The tightly wound fiber is also advantageous, when compared to relatively inflexible large-mode-area fibers by not exhibiting optical degradation or distortion and resulting in a more compact unit. The slowness and low repetition rate concerns with traditional Q-switches are also avoided since no Q-switch is needed or desired with the present laser apparatus. The present laser apparatus additionally has discrete sub-pulses clustered together within a very short time period, with each sub-pulse having an ultrafast (such as less than 100 femtosecond) duration, yet the clustered sub-pulses are insensitive to dispersion while increasing the energy delivered by the laser to the target without the need for amplification. An oscillator that results in high energy 0.1-1 uJ ultrafast pulses at 0.5-10 MHz is ideally suited for material processing, ablation and spectroscopy. Such ablation includes dental cornea and cataract surgery. Furthermore, the present laser apparatus is ideally suited for Laser-Induced Breakdown Spectroscopy (“LIBS”), selected Raman excitation, and endoscopy. Additional advantages and features of the present laser apparatus and method will become apparent from the following description and claims, as well as the appended drawings.